The water potential of the blood is kept within narrow limits by a homeostatic control system involving the hypothalamus, the posterior pituitary gland and the kidneys. Like all homeostatic systems it has receptors, a coordinator and effectors.Receptors. Osmoreceptors are cells in the hypothalamus that detect changes in the water potential of the blood. When the blood water potential falls (for example after sweating or eating salty food), water leaves the osmoreceptor cells by osmosis and they shrink. This stimulates them. When the blood water potential rises, they swell and are stimulated less.Coordinator. The osmoreceptors stimulate neurosecretory cells in the hypothalamus. These are neurones that make antidiuretic hormone (ADH) in their cell bodies. The ADH travels down their axons to the posterior pituitary gland, where it is stored. When the neurosecretory cells are stimulated, impulses pass down the axons and ADH is released into the blood.Effectors. The target cells are in the walls of the collecting ducts (and the late distal convoluted tubules). Their permeability to water changes, which changes how much water is reabsorbed.
change in blood water potential → osmoreceptors (hypothalamus) → neurosecretory cells → ADH from posterior pituitary → collecting ducts → change in water reabsorption This system links the nervous and endocrine systems: the hypothalamus is part of the brain, but its message to the kidney is carried by a hormone in the blood.How ADH acts on the collecting duct cells, and how feedback keeps water potential constant, are covered in the next lessons.